This invention relates to a beam shaping apparatus for laser diodes and, move particularly, to a device used for combining a plurality of laser beams emitted from a plurality of laser diodes to produce a single laser beam with high brightness.
As is well known in the state of the art, a conventional xe2x80x9chigh powerxe2x80x9d laser diode output beam has a relatively low brightness (brightness is equal to the ratio between beam power and product of beam area and beam solid angle) for many applications. In part, this is because the light emitting area of high power laser diodes has a very asymmetrical, rectangular form with typical width of approximately 1 xcexcm for most devices and a length of approximately 50-400 xcexcm. The long (i.e., length) and short (ie., width) axes are generally referred to as slow and fast axis respectively. In addition, the laser diode beam divergence is very different for the slow axis (full width at half maximum FWHMxcx9c8 degrees) and for the fast axis (FWHMxcx9c40 degrees).
However, for many practical applications of laser diodes it is necessary to have a spatially symmetric output beam with uniform divergence and increased power (i.e., increased intensity or brightness).
One method of obtaining a symmetrical output beam is by coupling a laser diode into an optical fiber, i.e., by fiber pig-tailing. A beam with increased brightness can be achieved by coupling more than one laser diode into the same fiber. The smaller the fiber core diameter, the higher the brightness at the output of the fiber pigtail. However, the asymmetrical form and divergence of a laser diode output beam require the use of an optical coupling system between the laser diode and the input fiber end. Two basic approaches were previously implemented to attempt to overcome this problem.
The first approach relies on a pair of asymmetrical polymorphic or cylindrical lenses to compensate spatial and angular asymmetries of the laser diode output beam and then uses a spherical lens to focus the radiation into a fiber core (see, for example, U.S. Pat. No. 5,321,718 to Lang et al. and U.S. Pat. No. 5,369,661 to Yamaguchi, both of which are incorporated herein by reference in their entirety). This approach only collimates the output beam of the laser diode, leaving the rectangular form of the beam substantially unchanged.
The second approach uses an optical apparatus to deflect the laser diode output beam wave front in various ways (to make it more symmetrical) to enable focusing of the radiation into a fiber. This optical apparatus can be implemented on its own or in addition to an asymmetrical lens pair. The second approach may be used with a single laser diode or in conjunction with several separate laser diodes, which are coupled into the same optical fiber.
An example of a device using several separate laser diodes was discussed in U.S. Pat. No. 6,075,912 to Goodman, incorporated herein by reference in its entirety. In the approach suggested by Goodman, the beams of several diodes are brought into close proximity via reflection at one common mirror. However, the Goodman system leaves the beam of each individual laser diode essentially undisturbed, resulting in practical limitations for a minimum spot size of a resulting output beam.
Another implementation of the second approach (U.S. Pat. No. 4,828,357 to Arata et al., incorporated herein by reference in its entirety) includes an apparatus producing a high power laser beam including (see FIGS. 1 and 2 herein) a plurality of lasers 101 and directing mirrors 102, a plurality of reflecting mirrors 203 and a central focusing mirror 204 for focusing the resultant laser beam into one focal point. Note that in this approach the polarization of the pump beam is not preserved. Moreover, generally, a plurality of reflecting mirrors is employed, complicating the setup. Equally, the beam of each individual laser diode remains essentially undisturbed, which produces practical limitations for the minimum spot size of the resulting output beam.
Yet another variation of the second approach is disclosed in U.S. Pat. No. 5,263,036 to De Bernardi et al. (incorporated herein by reference in its entirety) in which an improved efficiency of combining laser beams is achieved through the use of suitably positioned dichroic mirrors 301 and laser diodes 302 (see, e.g., FIG. 3 herein). The limitation of this technique is again that the beam of each individual laser diode remains substantially undisturbed which results in practical limitations on the minimum achievable spot size of the resulting beam. Moreover, the disclosure addresses direct launching of pumping radiation only into an active multimode optical waveguide containing an active monomode region, a technique which complicates the pumping arrangement.
U.S. Pat. No. 5,877,898 to Holleman et al. (incorporated herein by reference in its entirety) as depicted in FIG. 4 herein proposes to divide and recombine the collimated beams from several laser diodes to obtain a more symmetrical output beam. However, Holleman et al. rely on beam rotation to obtain an improved geometry of the output beam. As one skilled in the art would appreciate, beam rotation is very difficult to realize in practice. In this particular example several separate micro-optic elements are incorporated; i.e., first an optical beam is divided up into several smaller beams, then beam rotation is implemented for all of the individual beams and finally, the rotated beams are recombined before they are focused into an optical fiber. In the scheme by Holleman no provisions are made to obtain polarization sensitive operation. Moreover no explicit minimization of backreflections from the beam dividing and recombining optical elements is accomplished; i.e., the optical surfaces from the beam dividing and recombining elements are incorporated at an angle close to 90xc2x0 with respect to the input beam.
Another variation of the second approach is discussed in U.S. Pat. No. 5,825,551 to Neilson et al. (incorporated herein by reference in its entirety). As shown in FIG. 5 herein, the beam 501 from a single laser device is collimated and then sent through an optical xe2x80x98beam shapingxe2x80x99 apparatus 502. The beam shaping apparatus contains two substantially parallel reflecting surfaces 503, 504 to effectively rotate the extension of the laser device by 90xc2x0 while not rotating the orientation of the divergence angles of the output beam from the laser device. However, no provisions are made to minimize the beam size of a single broad stripe laser diode or to minimize the beam size of more than one laser device. Moreover, no provisions are included to optimize the transmission through the xe2x80x98beam shaperxe2x80x99 depending on the polarization state of the light incident to the beam shaper.
Accordingly, a need exists for an improved method of producing the smallest cross section from a laser beam and system for addressing laser diode asymmetries in both single and multiple laser diode systems.
An object of the invention is to provide a novel integrated arrangement by which the radiation of a plurality of laser diode light emitting areas is so combined and shaped that the combined beam bundle has a reduced and, preferably the smallest possible cross section with reduced and preferably the least possible asymmetry and far-field divergence. The beam shaping arrangement is further configured to minimize its size and insertion loss depending on the polarization state of the input beam from the plurality of laser diode light emitting areas.
This objective is met in an arrangement in accordance with the invention, i.e., an arrangement for combining and shaping the radiation of a plurality of laser diode light emitting areas comprising at least one laser diode whose radiation has a cross section in the emission plane (x-y plane) with a longitudinal axis which is much greater than in the transverse axis. The invention may further include for each laser diode a collimator unit in the radiation direction. A combining unit may be used to combine the collimated radiation of the individual laser diode light emitting areas by means of a side-by-side arrangement in the direction of the transverse axis. A low loss polarization sensitive single element recombining unit divides the combined radiation in the direction of the longitudinal axes into individual partial beams and recombines the latter by means of a side-by-side arrangement in the direction of the transverse axes.
The invention is based on the selection and sequence of the arrangement of optical means for collimating, deflecting, polarizing, backreflection suppression, dividing and combining the radiation from a plurality of laser diode output beams to form a beam bundle. For this purpose, the following optical elements and sub-elements (expressed in the form of optical means) may be arranged in the following sequence in the direction of primary radiation propagation:
Means for collimating the radiation of the laser diode output beams (e.g., a collimator unit);
Means for combining the collimated beams by means of a side-by-side arrangement in the direction of the transverse axis of the beams (e.g., a combining unit);
Means for dividing the combined radiation in the direction of the longitudinal axis of the radiation;
Means for polarizing the input beam;
Means for minimizing back-reflections; and
Means for deflecting the partial beam bundles in order to recombine these partial beam bundles in a side-by-side arrangement in the direction of the transverse axis (e.g., a recombining unit).
In order to illustrate the change in beam geometry, an x-y-z coordinate system is adopted herein. The laser diode light emitting areas have their emission surfaces arranged on a straight line in a plane with their longitudinal axes (see surface a of FIG. 6). The orientation of the longitudinal axes defines the y-axis and the orientation of the transverse axes defines the x-axis in the coordinate system. In the x-y plane, aligned with and defining the plane of the emission surfaces of the laser diode, the beam geometry is determined by the arrangement of the emission surfaces and accordingly presents a vertically oriented stripe. The horizontal width of the stripe corresponds to the fast axis of the laser diode, while the vertical height of the stripe corresponds to the slow axis. In the drawing, real proportion and dimension ratios of the beam cross-section are not maintained.
An object of the invention is to propose and provide an optical means to divide a collimated laser diode beam into a plurality of similar parts or portions and to form with these portions a more symmetrical beam geometry (symmetric beams B of FIG. 6) thereby optimizing the coupling of the radiation into an optical fiber. The distance between the two output beams may be adjusted for optimum symmetry. For two (or more) laser diodes in the module, a single optical means can be used to rearrange the input beams, increasing the number of output beams by a factor of 2 compared to the input beams and forming the symmetrical beam geometry (FIG. 7).